This invention relates to methods for temperature control in microwave processing, and in particular though not exclusively to such methods that enable temperature control during microwave heating, sintering or annealing. The invention further relates to methods for forming desired materials by microwave heating of precursor materials.
Microwave heating has been proposed for many thermal processing techniques such as heating, sintering and annealing because of its efficiency and speed, and also because of the fact that different materials have different microwave absorption properties leading to the possibility of new fabrication techniques. The major advantages of microwave heating are in particular a fast heating rate and selective heating. If the microwave frequency is chosen correctly, for example, it is possible to heat a sample to a high temperature within a container that is not susceptible to heating and therefore remains at low temperature.
For example, copper oxide can be heated from room temperature to over 1000xc2x0 C. in a matter of seconds by microwave heating at a frequency of 2.45 GHz. This can be used for example in microwave sintering of high temperature superconducting YBa2Cu3O7xe2x88x92x (Y123) the precursor of which includes approximately 27% by weight copper oxide. This can be carried out in an alumina container because alumina does not absorb at 2.45 GHz, but only at 28 GHz and therefore the container remains relatively cool. This selective heating property is particularly useful, for example, in thin film forming. For example, conventional heating makes it very difficult to heat thin films on a substrate because the substrate will also be heated which can lead to contamination of the thin film through diffusion of substrate components.
Unfortunately there are also some difficulties with microwave heating, and in particular with the control of the temperature during heating. Indeed even temperature measurement itself presents problems. For example, the temperature probe must not itself be susceptible to microwave heating at the same frequency as the samples being heated. Furthermore, if a metal probe is used it must be earthed or it may be damaged by arcing. In addition, since microwave absorption is material dependent, the heat generated by the microwave irradiation and hence the temperature achieved will vary with materials even if the same set-up and power is employed.
Current techniques for monitoring the temperature of samples during microwave heating include earthed thermocouples and infra-red pyrometers. For both methods, however, large errors are inevitable in comparison with conventional heating because of the larger temperature differences between the samples and the temperature sensing devices. Moreover, even if the temperature can be controlled, for example manually or by a complicated proportional-integrated-derivative (PID) control, temperature fluctuation can often be problematically large.
Some materials, such as NaH2PO4.H2O, reach a naturally stable temperature (in this case 820K) after dehydration, however the class of such materials having the potential for natural isothermal heating is small.
There remains a need for methods for temperature control in microwave processing that would facilitate microwave heating and mitigate some of the aforesaid problems and difficulties.
According to the present invention there is provided a method of processing a sample of material by microwave irradiation, wherein the sample is formed such that at least a part of the sample will reach a state of thermal equilibrium in which energy absorbed from the microwave irradiation is balanced by heat dissipated to the environment.
In particular the maximum temperature of the sample at which the state of thermal equilibrium is reached is controlled by varying sample parameters selected from the shape, dimensions, mass and density of the sample. In a preferred embodiment of the invention the sample is formed as a cone.
According to another aspect of the present invention there is also provided a method of forming a material by microwave heating, comprising the steps of:
(a) assembling precursor materials into a shape of selected form and dimensions, at least one of said precursor materials being susceptible of absorbing microwave radiation at at least one microwave frequency, and
(b) irradiating said precursor materials with microwave radiation at said at least one frequency,
wherein the form and dimensions of the shape into which the precursor materials are selected is chosen such that when said materials are subject to said microwave irradiation at least a part of said sample reaches a state of thermal equilibrium at which at least a part of said precursor materials are at a temperature sufficient to form said material.
Preferably the precursor materials are formed into the shape of a cone.